This invention relates to circuitry for the performance of Traffic Alert/Collision Avoidance Systems ("TCAS") commonly used in the aerospace industry, and more particularly to TCAS systems which are used in altitudes greater than fifty thousand feet.
Within the present discussion, the invention's attributes are described relative to the TCAS, but, the invention is not so limited and is intended to be applied to circuits which are to operate in very thin atmospheres.
TCAS is a high power pulsed radio frequency (RF) system with a transmit frequency of 1030 MHz. Internally, power levels can approach 1200 W.
Using the formula ##EQU1## where V.sub.rms is the root mean square RF voltage, P is the power level, and Z is the impedance presented at that location, a 1200 W power input equates to 245 V.sub.rms across 50 .OMEGA. impedance. The voltage levels can become much higher in areas of high impedance, resonant structures or where an impedance mismatch occurs.
Two phenomena which can be detrimental to high power systems are voltage breakdown (arcing) and corona. Voltage breakdown occurs when the applied voltage exceeds the breakdown voltage of the medium and essentially a short circuit is formed between the points across which the voltage is applied. This is generally a destructive failure mechanism which can result in charring of material and even can result in permanent short circuits.
Corona simply represents the ionization of a gas surrounding the point of high voltage. The gas usually is air, and corona results in a spectacular colorful discharge. Corona usually precedes arcing. The occurrence of corona can diminish the transmitted power level and as well can be an indicator of more significant problems at more extreme environmental conditions.
The existing art provides general improvements for reducing voltage breakdown and include such techniques as:
a) decreasing the applied voltage (by decreasing the impedance); PA1 b) increasing the separation between the points across which the voltage is applied; PA1 c) eliminating all sharp corners and transitions; and, PA1 d) eliminating all air gaps.
Generally all these improvements apply to reducing corona also, except that corona often occurs in open air instead of between two electrodes.
In a particular RF filter used to reduce harmonic output in a TCAS system, a susceptibility to corona has been noted. This filter is a distributed element filter in a microstrip design, meaning that the elements (inductance and capacitance) are printed in copper on a substrate. This filter is very sensitive to dimensional repeatability of the copper image and the material as well as the material properties like dielectric constant and loss tangent.
Unfortunately, the improvements listed above do not seem practical in the design of the TCAS filter. Decreasing the impedance implies larger microstrip circuitry, and circuit space is at a premium, plus the coupling to adjacent traces increases as the traces get closer, resulting in an electrical performance degradation. To achieve the necessary distributed element value for capacitance or inductance requires microstrip stubs of particular width and length, many of which are narrow lines with an abrupt end.
One way to eliminate all air would be to bury the circuit in a stripline assembly which surrounds the circuit with dielectric material. Although this is feasible, it makes the microstrip launch into the filter between layers much more difficult. Further, in the construction lay-up of this particular PWB, the filter becomes more lossy by the inclusion of FR4 material in the stripline assembly.
Also since the circuit is so large, the stripline method would use much valuable internal space otherwise reserved for interconnects.
Another option is to cover the filter with another layer of PTFE dielectric, called dielectric overlay. Dielectric overlay is difficult to model when using even the thinnest layers of PWB material, and the additional dielectric will significantly alter the performance of the filter.
Although corona is not a new problem, corona in high power RF circuitry used at high altitude (in excess of fifty thousand feet up to seventy thousand feet) has not been fully addressed. In this context, corona began to exist intermittently at 55,000 ft, at an input power level of 750 W.
It is clear that there is a need for circuitry which can operate in thin atmospheres without generating a corona or short circuits.